As such special effect system for providing special effects to television signals, there has been proposed one, for example, by U.S. Pat. No. 4,965,844, in which a television signal is processed to obtain a digital signal so as to display an image on a display, the image being seen just like an input image is pasted up (hereinafter referred to as "mapping") on a three-dimensional curved surface (for instance, cylindrical, like a wine glass, etc., hereinafter referred to as "shape").
In a method of this type of image conversion, the input image is divided into a plurality of blocks having a predetermined size, and image data of each block are stored progressively in predetermined addresses of a memory according to predetermined write address data.
At the same time, predetermined calculation is executed with respect to the address of the stored input image data prior to the conversion according to conversion input data input by the operator using separate input means, thus obtaining an address to read input image data stored in the memory.
The read address is calculated such that an output image just like an input image is mapped onto a predetermined shape assembled with a raster scan, and according to this read address image data in a block stored in predetermined address in the memory is read out. Thus, a two-dimensional plane output image, which appears just like the input image, is mapped onto a predetermined shape.
When actually mapping input image or video data VD.sub.IN shown in FIG. 1 into the shape of a wine glass, for instance, the operator inputs heights y.sub.0, y.sub.1, . . . in y-axis direction and corresponding radii r.sub.0, r.sub.1, . . . , as shown in FIG. 2, as shape data representing the wine glass by assuming the y-axis as the center of rotation, thus setting side contour S1 of the shape.
By rotating the side contour S1 about the y-axis, an address (x.sub.n, y.sub.n, z.sub.n) of typical point P of the shape S, as shown in FIG. 3, is calculated as shown by following equations: EQU x.sub.n =r.sub.0 cos.theta.(1) EQU y.sub.n =y.sub.0 (2) EQU z.sub.n =r.sub.0 sin.theta.(3)
In these equations, .theta. represents the angle with respect to the x-axis.
On the surface of the shape S thus calculated, pixels of each block of the input image VD.sub.IN are mapped as an inverse image, thus obtaining a two-dimensional plane output image VD.sub.OUT0 which looks just like the input image V.sub.IN, as shown in FIG. 4, mapped onto the surface of the shape S of the wine glass.
In the above image conversion method, however, the side contour S1 is set as the shape S and is rotated about the y-axis. In practice, therefore, it is possible to obtain only a rotor as the contour of the shape S, and it is impossible to meet an operator's demand of mapping the input image VD.sub.IN on various three-dimensional curved surfaces such as polygons. The method, therefore, is still inconvenient.